Coagulation factor VIII (FVIII) circulates in plasma at a very low concentration and is bound non-covalently to von Willebrand factor (VWF). During hemostasis, FVIII is separated from VWF and acts as a cofactor for activated factor IX (FIXa)-mediated factor X (FX) activation by enhancing the rate of activation in the presence of calcium and phospholipids or cellular membranes.
FVIII is synthesized as a single-chain precursor of approximately 270-330 kD with the domain structure A1-A2-B-A3-C1-C2. When purified from plasma (e.g., “plasma-derived” or “plasmatic”), FVIII is composed of a heavy chain (A1-A2-B) and a light chain (A3-C1-C2). The molecular mass of the light chain is 80 kD whereas, due to proteolysis within the B domain, the heavy chain is in the range of 90-220 kD.
FVIII is also synthesized as a recombinant protein for therapeutic use in bleeding disorders. Various in vitro assays have been devised to determine the potential efficacy of recombinant FVIII (rFVIII) as a therapeutic medicine. These assays mimic the in vivo effects of endogenous FVIII. In vitro thrombin treatment of FVIII results in a rapid increase and subsequent decrease in its procoagulant activity, as measured by in vitro assay. This activation and inactivation coincides with specific limited proteolysis both in the heavy and the light chains, which alter the availability of different binding epitopes in FVIII, e.g. allowing FVIII to dissociate from VWF and bind to a phospholipid surface or altering the binding ability to certain monoclonal antibodies.
The lack or dysfunction of FVIII is associated with the most frequent bleeding disorder, hemophilia A. The treatment of choice for the management of hemophilia A is replacement therapy with plasma derived or rFVIII concentrates. Patients with severe haemophilia A with FVIII levels below 1%, are generally on prophylactic therapy with the aim of keeping FVIII above 1% between doses. Taking into account the average half-lives of the various FVIII products in the circulation, this can usually be achieved by giving FVIII two to three times a week.
There are many concentrates on the market for the treatment of hemophilia A. One of these concentrates is the recombinant product Advate®, which is produced in CHO-cells and manufactured by Baxter Healthcare Corporation. No human or animal plasma proteins or albumin are added in the cell culture process, purification, or final formulation of this product.
The aim of many manufacturers of FVIII concentrates and therapeutic polypeptide drugs is to develop a next generation product with enhanced pharmacodynamic and pharmacokinetic properties, while maintaining all other product characteristics.
Therapeutic polypeptide drugs often are rapidly degraded by proteolytic enzymes and neutralized by antibodies. This reduces their half-life and circulation time, thereby limiting their therapeutic effectiveness. The addition of a soluble polymer or carbohydrate to a polypeptide, however, has been shown to prevent degradation and increase the polypeptides half-life. For instance, PEGylation of polypeptide drugs protects them and improves their pharmacodynamic and pharmacokinetic profiles (Harris J M et Chess R B, Nat Rev Drug Discov 2003; 2:214-21). The PEGylation process attaches repeating units of polyethylene glycol (PEG) to a polypeptide drug. PEGylation of molecules can lead to increased resistance of drugs to enzymatic degradation, increased half-life in vivo, reduced dosing frequency, decreased immunogenicity, increased physical and thermal stability, increased solubility, increased liquid stability, and reduced aggregation.
Thus, the addition of a soluble polymer, such as through PEGylation is one approach to improve the properties of a FVIII product. The state of the art is documented by different patents and patent applications:
U.S. Pat. No. 6,037,452 describes a poly(alkylene oxide)-FVIII or FIX conjugate, where the protein is covalently bound to a poly(alkylene oxide) through carbonyl-groups of said FVIII.
EP1258497B1 describes a method to prepare conjugates of FVIII and a biocompatible polymer. This patent was supplemented by a publication of Röstin et al. (Bioconj Chem 2000; 11:387-96). The conjugates comprise a B-domain deleted recombinant FVIII modified with monomethoxy polyethylene glycol. The conjugate had reduced FVIII function and the coagulant activity decreased rapidly with the degree of modification.
WO04075923A3 describes polymer-FVIII molecular conjugate comprising a plurality of conjugates wherein each conjugate has one to three water soluble polymers covalently attached to an FVIII molecule. The FVIII molecule is B-domain-deleted.
U.S. Pat. No. 4,970,300 describes a modified FVIII, wherein an infusible conjugate comprising a protein having FVIII activity was covalently linked to a nonantigenic ligand.
U.S. Pat. No. 6,048,720 describes conjugates of a polypeptide and a biocompatible polymer.
WO94/15625 describes FVIII bound to polyethylene glycol having a preferred molecular weight of no greater than 5,000 Daltons.
Nevertheless, there remains a need for improved FVIII having an attached soluble polymer to extend the half-life of the FVIII in vivo, which retains functional activity while providing an extended half-life in vivo, as compared to unmodified FVIII and other modified FVIII therapeutics known in the art.